This invention relates in general to stress relieving elements and more particularly to a thermal stress relief plate or substrate adapted to join heat generating elements, i.e. a semiconductor chip and a printed circuit board.
It is a continuing need as set forth by U.S. Pat. No. 4,427,993 in a number of fields to provide stress relief elements for electrically or mechanically coupling differing heat generating devices. This has been desired in that thermal stress can cause fatigue at the interfaces of these elements after repetitive expansion and contraction during thermal cycling, i.e. a shearing in soldered joints, as explained in U.S. Pat. No. 4,824,381.
Each of the aforesaid patents attempt to suggest a solution to the problem of thermal stress management, and it is in the improvement of their efforts that this invention disclosure is made. In that regard it is an object of this invention to provide a thermal coefficient of expansion matching substrate for joining members that during use will produce differing coefficients of thermal expansion whereby the substrate will keep joint stress low to maintain low bonding strains at each interconnect level.
It is yet another object of this invention to construct a substrate for interconnecting multiple elements whose coefficient of thermal expansion may be easily varied to match an element having a coefficient of thermal expansion in an area thereof and also match another element having another coefficient of thermal expansion in another area thereof.
It is still further object of this invention to provide a member that can be constructed to allow for selection of its coefficient of thermal expansion in the plane of its matching surfaces with other heat transmissive elements to allow joinder thereof.
An additional object of this invention is to provide a thermal coefficient of expansion matching substrate or mounting member having a plurality of matching surfaces each having areas of an independently selectable thermal coefficient of expansion for joining a like plurality of dissimilar elements.
In preferred embodiment of this invention a composite substrate containing Invar material is adapted to join a silicon chip to a printed circuit board so as to accommodate the variations of the chip's and board's coefficients of thermal expansion.